Alternator Secrets
That is a really interesting article on how to make an alternator output anything from 5VDC to 120VAC. So if anyone is thinking of making a hybrid setup like I was planning. Using electric drive as well as keeping the gas engine, one of the circuits described in that article lists how to make your own voltage regulator. So you can charge your 72V or whatever battery bank using the existing alternator.
One possibility is to periodically run the gas engine at an ideal speed to charge the batteries as efficiently as possible and drive on electric only so it acts as a serial hybrid. I am not sure if it would actually get you better mileage but once the alt is modified then it won't be hard to do a few experiments on what gives the best mileage overall. An alternator that can charge 12V as well as the higher voltage at the flip of a switch would give a lot more flexibility to what you can do.